Two-way signaling system



July 231929. J. F. FARRINGTON 1,722,031

TWO-WAY S IGNALING SYSTEM Filed Jan. 9 1924 mmn I John fart/hymn 17/ M Patented July 23, 1929.

UNETEE ST'EES L'IZZMI JOHN RFARRINGTON, or rLUsi-iine, new YORK, essrenoa 'r'o WESTERN ELECTRIC COMPANY, INCORPORATED, or new YORK, n. Y., A CORPORATION or new YORK.

TWO-WAY SIGNALING SYSTEM.

Application filed January 9, 1924. Serial No. 685,160.

The present invention relates to two-way signaling systems and more particularly to two-way signaling systems employing high frequency current.

This application is a continuation in part of Farrington application, Serial No. 482,644 filed December 23, 1920, which eventuated into Patent 1,566,469, granted December 22, 1925.

A two-way high frequency system, for the purposes of the present specification, is a system employing signal modulated carrier currents in which transmission and reception may occur simultaneously without too great interference andtoo great sidetone. Conversation can accordingly be carried on over such a two-way system in the same manner as in ordinary wire telephone systems employing only voice frequencies.

In signaling systems employing modulated waves it is common to provide a modulator by which the high frequency or carrier waves or currents are controlled in accordance with the signals to be transmitted and i a detector or demodulator for deriving from the received signal modulated wave, the current componentrepresenting the signal to be received. In such systems, balanced circuits are usually provided for making the detector neutral with respect to. the modulator so that the high frequency modulated energy from the output circuit of the modulator does not enter the local detector circuit. In U. S. patent to H. W. Nichols 1,519,626 granted December 16, 1924, a system is disclosed in which a single high frequency set is used for both modulating and detecting and this set is connected to the line or to the antenna through a single coupling or tuned circuit or filter, thus dispensing with high frequency balance.

In the system of Nicholss patent, detectiou occurs by reason of the curvilinear relation between the plate potential and space current of a three-electrode 'vaccum tube coupled to the antenna, this tube also being used to amplify the modulated current during transmission. It is important in such a system that the production of audible frequency interfering currents, during transmission, in the circuit containing the indicating device be kept within reasonable limits, not only for the comfort of theoperator but also to avoid injuring the indicating device or anamplifier in the channel which leads to it. Furthermore, it is desirable in a two-way1 system that the person speaking be able to above the sound produced in theindicator by these interfering currents. This enables the operator at the distant station .to interrupt the speaker asin an ordinary wire telephone conversation.

In such a syste1n,audible' frequency current may be produced in the indicator both on account of thesignal side tone resulting from detection of the impressed carrier and the signal modulated side bands, and on account of a beat current resulting from interaction between the incoming and outgoing carrier waves. Side tone would theoretically be avoided if a linear relation between the grid potential and space current and a curvilinear relation between the space potential and the space current of the ampli-. fier could be made independently to exist.-

An approach towardthe attainment of this result could be made with a tube which operated according toVan der Bijls equation I =K (E ,+;iE +Kz) ,if a tube were used which had a variable ,1. A means for sub stantially balancing out the signal side tone is disclosed in U. S. patent to Heising, Serial No. 1,464,097, granted August'Z, 1923. The interfering beat note resulting from the combination of. the currentsof two carrier frequencies can be avoided by accurately provide two-way signaling systems characterized by simplicity, low cost of construction and a minimum number of vacuum tubes. I 7

These objects are realized, in one modification, in a system similar to that described but in which the two carrier frequencies for opposite directions of transmissions are ear the signals from distant station broadly, of the invention is to.

constituted by modulator M, carrier current source'l, low frequency source 2 of the modulating currents, and their interconnected circuits, is coupled to antenna 3 throughthe' power amplifier A. The modulation system is of the type disclosed in U. S. patent to .Van derBijl, No. 1,350,752, granted August 24, 1920. Its function is to produce a wave having the frequency of the current from source 1, the amplitude of which varies in accordance with the current from source 2. The present invention is not dependent upon the employment of this or any other particular system of modulation. The arrangement is disclosed merely as a practicable example of a modulation system well adapted to be used with the present invention.

Current from the modulation system is impressed on amplifier A through transformer 4.-. Condenser 13'which prevents the flow of. direct current from space current source 14 in the primaryof transformer 4, is of such capacity as to'also effectively prevent the flow of amplified voice currents therethrough while interposing negligibleimpedance to thehigh frequency modulated currents. The output circuit of the amplifier is coupled through tuned circuit 9 and transformer G with branch 5 of the antenna. Branch 5 in combination with the aerial portion-of the antenna is tuned to the mean frequency of the outgoingmodulated carrier wave. Loop circuit 8 in the other branch 7 is anti-resonant at this frequency so as to effectively remove this branch from the antenna circuit so far asits function during transmission is concerned. p y

The output circuit of amplifier Ais completed through condenser 10 which should be of such capacity as to offer negligible impedance to currents having frequencies of the order of the outgoing carrier frequency.

carrier wave, the carrier frequencies for the two directions of transmission being chosen so as to differ by an inaudible amount.

During transmission, the system functions in a manner which needs little detailed discussion. Since telephone receiver 12 interposes a high impedance to the high frequency currents such currents will accordingly complete their circuit through the condenser 10 in shunt therewith. Voice frequency currents do not appear in the output circuit of the amplifier on account of condenser 13 in the output circuit of the modulator, as explained above. The receiver 12 accordingly will in no way be affected by the operations incident to signal transmission except to the" extent that the amplifier additionally functions as three-electrode detector to reproduce the signal wave in its output circuit from the impressed unmodulated and signal modulated carrier. The side tone produced in this manner is, in general, not useful and is usually suflicient to make the system undesirable for two-way operation, but it can be made of small volume by the method above mentioned, namely, by using a tube which amplifies without distortion, but which has a curvilinear relation of its space-potential and space current, or by the method of Fig. 2 to be described later.

During reception, the signal modulated. wave which is incident on antenna 3 is selectively received thereby and impressed on the plate-filament circuit of amplifier A. On account of the curvilinear relation between the space current and plate voltage of this circuit, detection will occur. The signal component resulting from detection will be inclicated in telephone receiver 12.

In practicable two-way systems of this general type, the carrier currents are continiu ously transmitted in both directions. Their received signal. During transmission or reception there will result a beat current in the receiver which, on account of the relatively great amplitude of the transmitted carrier current will be injurious to the'receiving instrument, and will give rise to a note which would be painful to the ear of the operator, and which might so completely drown out any simultaneously received signal as to render it impossible for a distant operator to break in on the transmission. During transmission this effect will, of course, be increased by the signal side tone, if present.

The detrimental effect of this beat frequency is avoided in the arrangement of this figure, without the introduction of undesirable effects, by using carrier frequencies which differ from each other by a frequency above the upper frequency essential for proper speech or other signal transmission, so that the telephone receiver diaphragm either cannot respond to the resultant beat frequency or so that current of thisfrequency can be separated from the current of signal frequency.

The alternative system of Fig. '2 is described in detail in applicants Patent 1,566,469, granted December 22, 1925. For this reason, and because the essential features which are common to the systems of the two figures have been described With reference to Figure 1, it will be sufficient to point out only the special features which distinguish the system of Fig. 2 from that of Fig. 1.

In Fig. 2 the elements having functions similar to the corresponding elements in Fig. 1 are similarly labelled. The common element in the transmitting and receiving circuits is oscillator O. This oscillator is of the type described in Colpitts United States application, Serial No. 214,971, filed February 1, 1918. The anode and control electrode of the electrode discharge device are connected to the circuit of the antenna in such a manner that the antenna itself constitutes the frequency determining circuit of the oscillator. Any other equivalent form of oscillator may be used. The particular form illustrated perhaps has special advantages in connection with the system of modulation with which it is associated, although with respect to the modulation system, as in the case of the oscillator, other equivalent systems" could be used.

The modulation system is of the form commonly denominated the constant current modulation system. It is described, for example, in United States patent to Sc'helleng No. 1,437,021, granted November 28, 1922. In this system the modulator or variable impedance tube M is functionally equivalent to an amplifier for the signal currents from source 2. The variation of current in the output of modulator M, resultingfrom the impressed signal variations, causes a corresponding variable potential drop in low frequency cho e 15 and consequently similar variations in the electromotive force impressed upon the plate of the oscillator tube, since the plate-filament circuits of the oscillator and modulator are supplied in common from direct current source 16 through the choke. These variations of plate potential are effective to cause corresponding variations or modulations of the generated carrier oscillations. Choke coil 17 has high impedance to the high frequency current from the oscillator and accordingly prevents them from reacting on the low frequency circuits. Its impedance is insuflicient at .low frequencies to interfere with the flow of signal frequency currents to the oscillator.

The received Wave is detected by the vacuum tube oscillator O to yield currents of the signal frequencies in its plate-filament circuit. These currents are impressed through transformer T on the low pass filter FL and pass through this filter FL and amplifier A,

to the indicator 12. The incoming and out going waves have. carrier frequencies differing by an inaudible amount. The use of the amplifier is optional. Its usefulness in this circuit depends upon the degree of amplifi cation otherwise obtained, as in the detector. The use of the filter FL is similarly optional.

If the beat frequency wave resulting from the combination of the incoming and outgoing carrier Waves is inaudible the elimination of this component is not required in order to avoid signal distortion. The use of the filter under these conditions would, however, result in more eiiicient indication of the signal, since the heatin and overloading effects ofthose beat frequency currcntsare thereby avoided. If the heat frequency is audible but above the essential signal fre-' quencies the filter is necessary inorder to separate the heatfrequency' and signal components. The system of Fig. 1 may, of

course, similarly include either one or both of these elements.

Since, in the particular method of modulation illustrated, bothincoming and outgoing signal currents must appear in the plate-filament circuit'of the oscillator-detector tube'the locally transmitted signals tend to cause interference in the local receiver. This is avoided in the system of Fig. 2 by causing the signal currents from the modulator M to flow in relatively opposite directions through coils L,

and L which together constitute the primaryv of transformer T. Artificial network N is designed, with relation to the coils L and L so that their effects are balanced in the trans.

in coil L During reception the path of the low frequency detected currents which flow through coilL iscompleted through the parallel paths which include, respectively, the

modulator tube M and the network N. The portion of this current which flows through coil L is in such a direction as to aid the effect of that flowing through the coil L Although the invention has been illustrated and described as embodied in a telephone system, it is not limited in application to such systems but may be applied 'to other types of signal systems. The essential features of the invention may be embodied in Widely different physical structures.

The scope of the invention is defined in the appended claims.

What is claimed is:

1. In a two-way high frequency signaling system, a device used in common for transmitting a high frequency wave and for detecting the modulating component of a received high frequency wave, said transmitted and received high frequency waves differing in frequency by an amount above the highest essential frequency of the signal, a transfer circuit for the detected modulating com onent, and means in said transfer circuit or preventing the flow therein of beat frequency current resulting from the intermodulation of the incoming and outgoing carrier Waves.

2. In a two-way modulated wave signaling system, a repeating device for transmitting a high frequency wave modulated in accordance with signals and for translating modulations of a high frequency received wave into signaling currents, a transmitter and a receiver for signaling frequency currents associated with said repeating device, the frequencies of the two high frequency waves differing by an amount above the highest essential signal frequency, and a means in energy transfer relationship with said receiver which is adapted to suppress currents of frequencies outside the range of signaling frequencies.

3. In a two-way system, a discharge device, a transmitting means for impressing high frequency. Waves upon said device for transmission, means for impressing received high frequency waves upon said device for detection, and a receiver and selective means associated with said device for receiving exclusively the detected signal components of the received wave, the frequencies of the respective high frequency waves differing by an amount at least higher than the highest essential signal frequency. a

4. In a two-way signaling system, a circuit for supplying high frequency modulated waves for transmission including a discharge device, said discharge device being designed to transmit high frequency waves and to detect received high frequency waves, and a telephone receiver and a filter associated with the output circuit of said discharge device for receiving a detected component of the received high frequency Waves to the exclusion of other frequencies, the frequencies of the transmitted and received high frequency waves differing by an amount above the high est essentlal signal frequency.

5. In a two-way signaling system, a threeelectrode electric discharge device for both.

the currents for transmission and reception differing in frequency by an amount above the highest essential frequency of the signal, and a receiving circuit connected to said receiving electrodes, comprising a band-pass filter having an upper cut-off frequency be-' tween the highest signaling frequency and said difi'erence frequency.

6. The method of two-Way high frequency signaling by means of a three-electrode electric discharge device having a cathode, an anode and an impedance controlling element whichconsists in utilizing such device as a three-electrode discharge repeater for transi'nitting a signal modulated carrier wave and independently as a two-electrode electric discharge detector for receiving a signal modulated carrier wave whose frequency differs by an amount outside the range of essential signal frequencies from that of the transmitted carrierwave, and in selecting the detected components corresponding to the signal currents to the exclusion of the component corresponding to said difference frequency.

7 The method of operating athree-elec trode discharge device having a cathode, an

anode and an impedance control element in two-way signaling which comprises impressing outgoing signal waves between said cathode and impedance controlling element and transmitting the resulting waves derived between the anode and cathode to a remote station, receiving from said remote station incoming signal waves differing in frequency from the outgoing waves by a frequency greater than the essential frequency range of speechcurrents, impressing said incoming signal waves between the cathode and anode of said device and deriving therefrom message indications whereby interaction of incoming and outgoing signals which might result in noise currents which would interfere with the proper signal indications is prevented.

8. In a two-Way signaling system, means for receiving signal waves incoming from a remote station, means for transmitting to said remote station signal waves differing from said incoming waves by a frequency exceeding the highest essential frequency .1

speech, and an. electron discharge device aving a cathode, an anode and an impedance control element, said cathode and control element having a connection to said wave transmitting means, said cathode and anode having a connection to said signal wave receiving means whereby said electron discharge device may transmit the outgoing signal waves and may detect such received signal waves without producing any intermodulation component within the voice frequency range.

9. In a. two-Way system for transmitting low frequency signals, means for receiving from a remote station waves representing low frequency signals, means for transmitting to said remote station Waves representing low frequency signals but differing in frequency from the received Waves by an amount in excess of the highest essential frequency of said low frequency signals, an electron discharge device having a cathode, an anode and an impedance control element, a circuit between said cathode and anode, means for impressing the received waves upon said circuit whereby they may be demodulated by said device operating as a two-element dischargedevice, a circuit connecting'said cathode and impedance control element, means for 'impressing said outgoing waves upon said lastnamed circuit whereby the production by said device of intermodulation components falling within the essential frequency range of said low frequency signals is precluded.

In witness whereof, I hereunto subscribe my name this 5th day of January A. 1)., 1924.

JOHN. F. FARRINGTO'N. 

